The present invention pertains to ovens for drying containers and, more particularly, to so-called pin ovens for drying open-ended cans.
Pin ovens are well known in the art and are widely used in the industry for drying the coating on the exterior of partially completed, open-ended beverage cans. The coating applied to the exterior of the cans may include the ink or enamel used to apply the label, an overcoat of lacquer or varnish, or both a printed label and overcoat. U.S. Pat. No. 3,381,391 shows a basic pin oven construction which has remained essentially the same and is in current use throughout the industry. A typical pin oven includes a conveyor chain mounted for movement in a generally vertical serpentine path defined by a series of straight runs connected by curved turn single sprocket sections. Extended carrier pins are attached to the conveyor chain in spaced relation along its entire length and open-ended cans are placed onto the extended pins and are carried over the serpentine path through the oven. Nozzles aligned with the chain path and the cans direct heated air against the outsides of the cans as they travel through the oven.
The heated air streams are also intended to help hold the cans on the chain pins and, therefore, most pin ovens include nozzle arrangements which continuously direct heated air against the can bottoms. In addition, attempts have been made to utilize the flow of heated air against the can bodies to stabilize the cans on the pins and to prevent the cans from flopping on the pins as a result of air flow variations or centrifugal force as the cans pass around the curved turn sections between adjacent vertical runs. U.S. Pat. No. 4,662,085 shows a more recent pin oven construction intended to promote uniform and rapid drying, while eliminating can flopping or flutter over the path of movement through the oven. This patent discloses the use of heated air streams directed angularly along the can side walls, as well as against the can bottoms. The pin oven disclosed in this patent is described as having a maximum capacity of 1,500 cans per minute.
Attempts have also been made to reduce can residence time in the oven and simultaneously reduce the oven size by substantially increasing the temperature of the drying air. In prior art device, utilizing a chain speed sufficient to process 1,500 cans per minute, drying air temperatures up to about 750.degree. F. (400.degree. C.) were used. However, line stoppage in pin oven systems is a common occurrence and, when cans are held at such elevated temperatures, they begin to soften and distort and create serious quality problems.
U.S. Pat. No. 4,053,993 shows another approach which is intended to eliminate the problems attributed to pin ovens, particularly the inability to adequately stabilize the cans and to direct drying air to the can interiors as well. In the system of this patent, cans are conveyed open end down on a perforated or open conveyor belt and gentle flows of air are imparted from above and both sides through the walls of a three-sided duct through which the conveyor and cans move. A part of the flow of heated air is caused to circulate upwardly through the open conveyor and into the can interiors. However, because the cans are standing on their open ends without any supporting pins or other devices extending into the can interior, the heated air flows must of necessity be of low velocity, particularly where a part of the flow is directed from below into the can interior. This is also a high temperature system and, when used to process a single line of cans, is only capable of handling approximately 1,000 cans per minute.
It is also known to dry inks and finishes applied to the exterior of cans in pin ovens in which the pins include holding means which contact the entire cylindrical interior of the cans and hold them centered on the pins as the pin chain travels through the oven. Means are also provided to cause the pins to rotate on their axes, thereby causing the cans to also rotate as they travel through the oven for more uniform drying. One version of such a drying oven is shown in U.S. Pat. No. 4,052,152 which includes an embodiment in which the can is not held centered on the pin and is dried by gas flames made to directly impinge on the can bottom and side walls. This arrangement is not known to have attained any measure of commercial success because of the propensity to burn the coatings and inability to attain can drying uniformity.
Can manufacturing plants utilizing pin ovens typically operate around the clock, seven days per week. Energy consumption, both in terms of natural gas used to fuel the air heaters and electricity to operate the blower motors, is very substantial. Energy cost savings are, therefore, extremely important. Thus, the ability to process cans in greater volumes with lower residence time in the oven and without increasing air temperature above a desirable maximum of about 415.degree. F. (213.degree. C.), would provide a substantial saving in energy costs. In addition, reducing the amount of natural gas used in the air heaters will also substantially lower the level of potentially hazardous emissions of NO.sub.x and carbon monoxide.
Finally, conventional pin ovens of the type initially described hereinabove have now reached practical limits of can drying speeds and capacities. Thus, current limits on pin oven capacities of about 1,500 cans per minute are too low to accommodate the higher capacities of improved can decorating equipment and other can manufacturing and handling equipment served by the pin oven.